iconicaapl
mBlox, OpenMarket, Mobile365 all tried to implement the half assed MMS implementation forced on them by the carrier market and it was an absolutely nightmare. I'm surprised noone in the tech press ever did a deep dive expose on just how archaic the whole certification process was, and the user experience was terrible.
Summary: this is not the first time the carriers have opened up MMS. Hopefully it will be the first time developers can leverage MMS the way it was intended.
EDIT: I realized that I didn't actually answer your question. MMS can be supported by carriers in different ways but the most typical interface is MM7 which is SOAP over HTTP. There are many MM7 gateway vendors and they all handle MIME boundaries, payloads, transcoding, etc different - subtle differences, but enough to make testing against a large matrix of devices very, very hard.